In recent years and continuing, optical disks (e.g., CDs (Compact Disc) and DVDs (Digital Versatile Disc)) serving to record computer programs, audio information, video information (hereinafter referred to as “contents”) are drawing greater attention owing to the advances in digital technology and the improvements in data compression technology. Accordingly, as the optical disks become less expensive, optical disk apparatuses for reading out the information recorded in the optical disks have grown to become widely used.
The amount of information to be recorded in the optical disks is growing year by year. Therefore, further increase in the recording capacity of a single optical disk is expected. As for measures that are being developed for increasing the recording capacity of the optical disk, there is, for example, increasing the number of recording layers. Accordingly, vigorous research is being made on optical disks having plural recording layers (hereinafter referred to as “multilayer disk”) and optical disk apparatuses that access the multilayered disks.
In the multilayer disks, there is a possibility that the signals from a target recording layer be adversely affected by spherical aberration if the spaces between the recording layers are too large. Accordingly, there is a trend of reducing the space between the recording layers. However, reducing the space between the recording layers causes cross-talk between the recording layers (so-called “interlayer cross-talk”). As a result, the beam returning (reflected) from the multilayer disk contains not only desired beams reflected from a target recording layer (hereinafter referred to as “signal light”) but also a significant amount of undesired beams reflected from recording layers besides the target recording layer (hereinafter referred to as “stray light”). This leads to the decrease in S/N ratio of reproduction signals.
For example, FIGS. 50A and 50B are schematic drawings for describing an operation of reading out information from a dual layer recording medium. FIG. 50A is a ray diagram showing a case of reading information recorded in a first recording layer L′ 0, and FIG. 50B is a ray diagram showing a case of reading information recorded in a second recording layer L′ 1 (See also FIG. 2).
In FIG. 50A, the objective lens 104 is positioned away from the substrate surface to form a fine beam spot on the first layer L′ 0. In FIG. 50B, the objective lens 104 is positioned closer to the substrate surface to form a fine beam spot on the second layer L′ 1. As shown in both FIGS. 50A and 50B, the signal light rays reflected from the first and second layers L′ 0, L′ 1 are changed to parallel rays when they are transmitted through the objective lens 104, and are condensed and detected at the same light reception surface 108 if the detection lens 106 is arranged at a fixed position.
FIG. 51 shows the results observing the degradation of jitter of the signal reproduced from the first layer MB0 in a case of reducing the thickness of an intermediate layer between the first and second layers MB0 and MB1 of a dual layer DVD disk.
In a case of reading out information from the first layer MB0, stray light is generated from the second layer MB1, as shown with the dotted lines in FIG. 51A. In a case of reading out information from the second layer MB1, stray light is generated from the first recording layer MB0, as shown with the dotted lines in FIG. 51B. A portion of the stray light overlaps with a beam reflected from the target recording layer and is detected at the optical detector 108.
This stray light is generally detected as the offset for various signals (described in further detail in “Analyses for Design of Drives and Disks for Dual-layer Phase Change Optical Disks”, pp. 281-283, Shintani et. al).
Furthermore, in a case of reducing the thickness of the intermediate layer, interference between the signal light and the stray light before reaching the optical detecting unit 108. This interference creates noise components for focus error signals, track error signals, and disk reproduction signals (jitter). For example, in observing the jitter of the signals reproduced from the first recording layer MB0, FIG. 52 shows that the jitter is adversely affected when the intermediate layer is formed with a thickness less than 30 μm. This phenomenon is typically referred to as cross-talk. Accordingly, in a case of reducing the thickness of the intermediate layer of a dual layer recording medium, it is desired to eliminate or reduce the stray light in an optical pickup apparatus.
In one related art example, offset caused by stray light may be eliminated by providing a diffraction grating in an optical detecting system for dividing the signal light and the stray light into primary light and secondary light, detecting the stray light from plural layers with different optical detectors, and calculating the difference between the signal light and the stray light (see Japanese Laid-Open Patent Application No. 2001-273640). However, with this related art example, not only is the stray light diffracted by the diffraction grating but the signal light is also subjected to the diffraction. This causes loss of signal light components included in the beam reflected from the optical disk. Furthermore, this related art cannot eliminate the changes in the quantity of light caused by the interference between the signal light and stray light prior to reaching the optical detecting surface, to thereby cause the strength of the signal light to vary.
In another related art example, the effects of the stray light may be reduced by providing a condenser lens and a pin hole in an optical detecting system (see Japanese Laid-Open Patent Application No. 2003-323736). However, with this related art example, the strongest component of the stray light may pass through the pin hole and be detected by the optical detector. Therefore, detection of the stray light cannot be sufficiently prevented. Furthermore, since the objective lens typically is driven in the tracking direction, deviation of the optical axis is likely to occur. In such a case, the signal light may be blocked due to the position of the pin hole, to thereby lead to a change in the strength of the signal light.
As another related art example, Japanese Registered Patent No. 2624255 proposes an apparatus for reducing interlayer cross-talk when reading out from a multilayer disk.
This apparatus requires to further reduce the diameter of a pin hole of its detector for reducing the components of the stray light that is incident on the detector. However, reducing the diameter of the pin hole also causes loss of the components of the signal light that is incident on the detector.